One of the advantages of electrostatic spraying is that the charged spray tends to wrap around the target. This can be of particular use in, say, agricultural spraying because the spray will cover both sides of the leaves of a plant, not merely the outer or upper surfaces as would be achieved with conventional spraying. Another feature is that the attraction of the spray to the target may reduce the amount lost by drift. More or most of the spray reaches its intended target. This reduces the total amount of spray which has to be used which reduces the cost of effective treatment and is thought to be generally better for the environment.
Electrostatic spraying apparatus is known in which a spray head has a spraying edge, an electrically conducting or semiconducting surface and means for delivering liquid to be sprayed to the edge via the surface; an electrode spaced from the edge; and high voltage supply means for generating a high voltage between the surface and the electrode so that, in use, when covered by the liquid to be sprayed, the electric field strength at the edge is intensified sufficiently, that the liquid at the edge is drawn out preponderantly by electrostatic forces into ligaments which break up into electrically charged droplets.
An apparatus falling within this broad type is disclosed in British patent specification No. 1569707.
An advantage of this apparatus is that the ligaments break up into droplets having a very narrow spectrum of diameters. This is preferred because if a droplet of a particular size is required to carry a lethal dose of an insecticide, say, smaller droplets are wasted as ineffective while larger droplets require a larger amount of insecticide to provide the same number of sites.
In order to treat large areas spraying can be effected from aircraft. Although aerial electrostatic spraying has been proposed, e.g. European patent application No. EP-A1-186353, a problem which has not been addressed is that caused by the airstream past the aircraft. In fixed wing aircraft used for spraying, there is an airstream past the vehicle due to its movement and possibly accentuated by the slipstream from a propeller, of the order of 70 mph. The problem cause by the airstream, is that turbulence around the electrostatic spray head interferes with the formation of the ligaments and thus spoils the spectrum of droplet diameters or even prevents spraying.